With a Mexican navy helicopter whirring low over a rural community on the U.S. side of the Rio Grande, an American military veteran armed with marksmanship skills and a hefty rifle hankered from the ground to shoot it down.

"Don't do it," Zapata County Sheriff Sigifredo Gonzalez Jr. recalled warning the man on that Sunday afternoon last year.

The flight is one of as many as 10 in the past 17 months in which South Texans - in broad daylight - have spotted Mexican helicopters hovering overhead. The aircraft was so close to the ground military personnel could be seen inside, Gonzalez said.

While an array of U.S. federal and state agencies declined to comment, a Mexican government official confirmed that Mexican military helicopters have permission to use Texas as a staging ground for missions into Mexico to fight drug traffickers.

"Yes, I can tell you they exist, they are going on," said the official, who has knowledge of the flights. "Certainly, for the last couple of years," he said, noting that the U.S. government also has permission to fly unmanned surveillance planes in Mexico.

Most sightings have been in the vicinity of Falcon Lake, a region where authorities in Mexico are fighting the Zetas cartel.

In some instances, American civilians snapped photos of the flights and shared them with police. In the most recent incident last month, a Mexican military helicopter landed at Laredo's airport.

Although apparently rare, permission to allow Mexican government helicopters to fly out of the United States goes back at least until 1987, when Mexican federal police flew in from West Texas to attack the legendary drug boss, Pablo Acosta, a marijuana kingpin.

U.S. Rep. Michael McCaul, who represents part of Harris County and is chairman of the Homeland Security Oversight and Investigations Subcommittee, declined to confirm details about the Mexican military's involvement with the United States.

"It is a highly sensitive, coordinated effort to take out the cartels that has been effective," he said. "Beyond that I don't want to compromise the operations or the safety of our agents."

Gonzalez, who testified before a congressional committee in May about border security, said he doesn't know what he can tell the citizens of Zapata County.

The massive Rio Grande reservoir is where an American jet skier was reportedly killed last year while fleeing Zetas cartel gunmen in boats. His body was never found.

"Given the corruption in Mexico, how do I know what these guys are doing, dropping something off, picking some thing up – kidnapping somebody and taking them back to Mexico?" the sheriff asked.

"Citizens have complained," he said. "What do we do, shoot them down? Can you imagine someone saying, 'This is my country, I'm protecting it.' "?

Robert Kline, a retiree who lives part of the year in Zapata, said that in March 2010, the thump of helicopter rotors moving over Falcon Lake from Mexico startled him.

"I am standing on the deck looking at it. 'Holy man, that's not one of ours,' " he recalled.

On the helicopter was Marina, the word the Mexico uses for navy.

Catherine Frazier, deputy press secretary for Gov. Rick Perry, said state law enforcement is not being told in advance of any operations in which Mexico's military has permission to fly over Texas.

"Failure to do so stirs up unnecessary concern among local people, and could result in situations of mistaken identity or friendly fire, putting our peace officers and citizens in harm's way," she said.

"The Mexican government does not comment on operational characteristics of its efforts to confront transnational organized crime," he said.

In years past, the U.S. government has labeled some flights as "incursions," or unauthorized border crossings.

The conservative group, Judicial Watch, which specializes in obtaining government documents, has posted reports listing incursions by Mexican government employees, including at least eight helicopters in 2007, apparently the last year in-depth descriptions were available.

Judicial Watch President Tom Fitton said he can't imagine the Mexican military being permitted to fly over U.S. soil.

"There are always excuses that can be made for flying helicopters around …but to have authorized activity of that nature would be explosive," he said, adding it would "make the Fast and Furious program seem like a tempest in a tea pot."

'Very disturbing'

Fast and Furious was an operation in which federal agents let guns slip into Mexico as part of a plan to track them to cartels. But the weapons were lost and some later turned up at the scene where a Border Patrol agent was killed.

U.S. Rep Ted Poe, whose district stretches from Spring to Beaumont, asked for an explanation for at least one flight which occurred over Starr County, not far from Falcon Lake.

"My office contacted Homeland Security and they confirmed that there was an unauthorized flight over Falcon Heights that day, but they said it was impossible to confirm whether or not it was actually a Mexican military helicopter or the drug cartels flying in a helicopter painted to look like a Mexican military helicopter," he said, "The Mexican military denies that they had any military helicopters flying the area that day."

Neither scenario is acceptable, he said.

"It is very disturbing that here are incursions into American air space by any aircraft originating from foreign countries and no one seems to be held accountable."